After he made some dubious claims about the company, fewer people trusted him.
They were renowned as people of dubious morals.
1960 March, H. P. White, “The Hawkhurst branch of the Southern Region”, in Trains Illustrated, page 173:
There are several reasons why the line's future is now dubious. With the exception of Horsmonden, the stations are most inconveniently sited. This is largely because the Wealden villages were built on ridges, and in fact most Wealden stations are badly placed.
2011, Nigel Jones, "A Tale of Two Scandals", History Today, February 2011, Vol. 61 Issue 2, pages 10–17
Evasive, womanising, boastful, malicious, untrustworthy, an inveterate gambler who combined his mediocre military career with running a high-class brothel, permanently cash strapped and viciously quarrelsome, his character is as dubious as his unsavoury appearance.
2018, James Lambert, “A multitude of ‘lishes’: The nomenclature of hybridity”, in English World-Wide, page 2:
Urban Dictionary records at least 66 of the terms found by the present research, but as this dictionary liberally accepts words, definitions, and sample sentences based solely on the say-so of contributors, in the absence of corroboration from other sources the authenticity of some entries must remain dubious.
She was dubious about my plan at first, but later I managed to persuade her to cooperate.
2010, John M. Broder, "Global Climate-Change Talks Begin in Cancun With More Modest Expectations", New York Times, November 30, Section A, Column 0, Foreign Desk, page 12
Last year, President Obama had large majorities in Congress and hopes of passing a comprehensive climate and energy bill. Next year, he faces a new Congress much more dubious about the reality of climate change and considerably more hostile to international efforts to deal with it.
Although largely synonymous with doubtful, there is a notable difference when describing statements or facts: dubious is used when the statement is a cause of doubt, while doubtful is used when a fact is in doubt. For example, “the company’s earnings report was dubious” (questionable) versus “his chances for recovery are doubtful” (uncertain or unlikely).